Friday, 27 May 2016

1709 - Why was it so cold?

The three-hundredth anniversary of the Grand hiver / Great Frost in 2009 generated particular interest because it coincided with a major new initiative in the study of historical climatology. The "Millenium" paleoclimate project, which was launched with EU funding in 2006 and ran to 2010, aimed to use the lastest scientific methods to piece together a thousand years of precise pan-European climate data. The objective was not further historical research per se but to establish definitively whether 20th-century climate could be considered to fall within the parameters of natural variability. The project involved 45 European participants, from Great Britain, Central Europe and Switzerland - though not for some reason from France - and was co-ordinated by Professor Danny McCarroll from the University of Wales at Swansea. In 2009 Professor McCarroll and his colleague on the project Dennis Wheeler, Reader in Geography at the Sunderland University, featured on the Radio Four programme Material World discussing the Great Frost/"Grand Hiver" of 1709. They had some interesting things to say.

The presenter Quentin Cooper began by asking whether the freezing conditions of 1709 were really so exceptional. Dr Wheeler admitted that (in England at least) the worst winter on record was in 1683-4 which according to the archives was even colder in terms of depth and duration of frost. But 1709 was a "pan-European event of the first magnitude" and remarkable by any standards, particularly since it occurred in a period when the climate was for the most part getting warmer. Danny McCarroll pointed out that it was also exceptional in its geographical extent; Europe froze all the way from Scandinavia to Sardinia.How cold was it?Although the project team has developed a whole series of sophisticated "proxy-indicators" of climate change - tree rings, lake sediment cores, peat cores, banded marine shells - they still relied heavily on contemporary records. By 1709 the thermometer was less than a hundred years old; the instruments were not standardised; there were even different scales in use. Nonetheless they give a clear indication of prolonged periods of sub-zero temperatures. Carefully written notes often give us an idea of what was meant by a given reading.

Professor McCarroll comments that he was "amazed" by the degree of international correspondence and the careful comparison of observations that existed at this period. We are here very much in the world of the Early Enlightenment. There were large numbers of people - not particularly British, says Danny McCarroll despite the prompting - clerics, medics and other amateurs scientists, who were happy to make observations of the world around them. "We are entering a world in which there is much more rational debate and a clearer scientific understanding. And people were beginning to ask questions about why the climate varied the way it did. Just six years earlier, in 1703, Britain and some parts of Western Europe had experienced the so-called Great Storm. That was often described as being a visitation by God to remind the people on earth of their sins. As you move through the early 18th century, you move out of that religious interpretation into a more rationalist scheme of thought. So it is not only interesting climatologically; the period is interesting from the point of view of the history of science and philosophy"

A clearer idea of temperature can be gained from records giving specific physical evidence, for example at what point lakes at a given altitude in the Alps froze. In Stockholm Harbour every ship entering or leaving had to pay a tax; extant registers allow us to determine the first day each year when the harbour was thawed enough to allow the movement of ships. This has formed the basis of a 500-year series.

Dennis Wheeler is the doyen of historical climate research based on ships' logs. As he explains, these are particularly valuable as there is otherwise little data for maritime areas. The Admiralty imposed an obligation on British Navy ships to make daily records of activities on board ship, including observations of wind strength and direction and copious notes on general state of the weather. There are over 100,000 RN logbooks for the period 1680 to 1850. Dr Wheeler has compiled a complete series back to 1685 for the English Channel area.

Explanations - or lack of.....

Dr Wheeler was then asked if he had formed any theories about the reasons for the Great Frost of 1709. It turns out this is something of a mystery. Periods of extended cold are normally associated with long runs of easterly weather during the winter, bringing cold air from Siberia and Eastern Europe into Western Europe The logbooks do not give clear picture of easterly winds, nor do the contemporary accounts. William Derham from Upminster in Essex, makes frequent reference to the variability of the winds. So this is a cold winter which doesn't quite conform to what might be expected from present-day scientific understandingThe two meteorologists are then asked about a couple of general theories. The first is the effect of the sun's activity. (It is known that the cold period c.1645-1715 coincided with a period of low solar activity, the "Maunder Minimum", signified by an absence of sun spots) The influence of solar activity on climate is contentious, but Dr Wheeler is non-committal: the new evidence does not contribute significantly to the sunspots debate; climate is a highly complex system, and the modern climate is distorted by greenhouse gasses. Another factor, often cited in French sources, is the influence of volcanic activity - there were four significant eruptions in 1707-8 (including Mount Fuji, Santorini and Vesuvius) in 1707-8. Danny McCarroll is more categorical here: volcanoes do have a big effect on climate, but they generally create a cooling effect in the summer not the winter; often the winters are slightly warmer. He concludes, honestly, that he has no answers to the causes of the Great Frost. 1709 falls in a period where there were quite a lot of cold winters. But the severity and extent of the cold weather, when not dominated by easterlies, is"a complete mystery to me".

: ". ..."Something unusual seems to have been happening," says Dennis Wheeler, a climatologist at the University of Sunderland, UK. As part of the European Union's Millennium Project, which aims to reconstruct the past 1000 years of Europe's climate, Wheeler is extracting data from Royal Navy logbooks, which provide daily observations of wind and weather. "With daily data you can produce very reliable monthly averages but you can also see what happened from one day to the next," says Wheeler. He and his colleagues have now compiled a database of daily observations stretching back to 1685 from the English Channel area. "This is a key climatic zone. The weather there reflects wider conditions across the Atlantic, which is where in normal circumstances much European weather originates."The most immediate cause of cold winters in Europe is usually an icy wind from Siberia. "What you would expect would be long runs of easterly winds with a well-developed anticyclone over Scandinavia sucking in cold air from Siberia," says Wheeler. Instead, his data show a predominance of southerly and westerly winds - which would normally bring warm air to Europe. "There were only occasional northerlies and easterlies and those were never for more than a few days," says Wheeler. Another odd finding was that January was unusually stormy. Winter storms tend to bring milder, if wilder, weather to Europe. "This combination of cold, storms and westerlies suggests some other mechanism was responsible for that winter."There may be no easy explanation for the Great Frost of 1709, but unexpected weather patterns revealed by Wheeler's data underline why climate reconstructions are so important. "We need to explain the natural variation in climate over past centuries so that we can tease apart all those factors that contribute to climate change. But before we can do that we need to nail down those changes in detail," says Wheeler. "Climate doesn't behave consistently and warmer and colder, drier and wetter periods can't always be explained by the same mechanisms." In the two decades after that terrible winter, the climate warmed very rapidly. "Some people point to that and say today's warming is nothing new. But they are not comparable. The factors causing warming then were quite different from those operating now."."]